JP3836373B2 - Magnetic recording medium - Google Patents

Abstract

A magnetic recording disk has a magnetic recording layer formed on a special multilayered "host" layer. The host layer is a "synthetic antiferromagnetically", i.e., at least two ferromagnetic films that are exchange-coupled antiferromagnetically (AF) to one another across a nonferromagnetic spacer film so that their magnetic moments are oriented antiparallel. The magnetic recording layer has a different composition from the top ferromagnetic film in the host layer and is ferromagnetically coupled to the top ferromagnetic film of the host layer. The magnetic volume V of the composite structure (magnetic recording layer and host layer) that determines the thermal stability will be approximately the sum of the volumes of the grains in the magnetic recording layer and the AF-coupled ferromagnetic films of the host layer. However, the magnetic moment of the composite structure is primarily just the moment from the magnetic recording layer because the host layer is designed to have essentially no net magnetic moment.

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention generally relates to magnetic recording media.
[0002]
[Prior art]
Conventional magnetic recording media, such as hard disk drive magnetic recording disks, generally use a granular ferromagnetic layer such as a sputter-deposited cobalt-platinum (CoPt) alloy as the recording medium. Each magnetic domain of the magnetic recording layer is composed of many small magnetic particles. The transition of the magnetic section represents a “bit” of the recorded data. U.S. Pat. Nos. 4,789,598 and 5,523,173 describe this type of conventional rigid disk.
[0003]
As the storage density of the magnetic recording disk increases, the product of the residual magnetization Mr (magnetic moment per unit volume of the ferromagnetic material) and the thickness t of the magnetic layer decreases. Similarly, the coercivity field or coercivity (H _c ) of the magnetic layer increases. This results in a reduction in the ratio Mrt / H _c . In order to achieve a reduction in Mrt, the thickness t of the magnetic layer can be reduced, but only to a certain extent. This is because the magnetization in the layer is susceptible to thermal collapse. This decay has been attributed to thermal activation (superparamagnetic effect) of small magnetic particles. The thermal stability of the magnetic particles is determined mainly by K _u V. Where _Ku is the magnetic anisotropy constant of the magnetic layer, and V is the volume of the magnetic particles. As the thickness of the magnetic layer decreases, V decreases. Therefore, if the thickness of the magnetic layer is too thin, the stored magnetic information is no longer stable under normal disk drive operating conditions.
[0004]
One approach to the solution to this problem is to move to high anisotropic materials (high _Ku ). However, the increase in K _u is, K _u / Mr approximately equal coercivity H _c in is the limit is a point which is too large to write the conventional recording head. A similar technique is to reduce the Mr of the magnetic layer by fixing the layer thickness, but this also limits the coercivity that can be written. Another solution is to increase the interparticle exchange so that the effective magnetic volume V of the magnetic particles is increased. However, this technique has been found to degrade the intrinsic signal to noise ratio (SNR) of the magnetic layer.
[0005]
Our co-pending US patent application Ser. No. 09 / 416,364, filed Oct. 8, 1999, anti-ferromagnetically couples a conventional single magnetic recording layer across a non-ferromagnetic spacer film. The problem of thermal stability is addressed by replacing the two ferromagnetic films. Since the magnetic moments of the two antiferromagnetic coupled films are oriented antiparallel, the net residual-thickness product (Mrt) of this recording layer is the difference between the Mrt values of the two ferromagnetic films. However, this reduction in Mrt is achieved without a reduction in the thermal stability of the recording layer (expressed in K _u V). This is because the volume V of the particles in each of the two separate antiferromagnetic coupling films is added so as to strengthen each other. Although this approach seems promising, a new set of unknowns regarding the magnetic and recording / reproducing characteristics of this unconventional recording layer is introduced.
[0006]
[Problems to be solved by the invention]
Magnetic recording that supports extremely high density recording while maintaining good thermal stability, and can use the well-known magnetic characteristics and recording / reproducing characteristics of conventional magnetic recording materials such as single layer granular Co alloy magnetic materials A medium is sought.
[0007]
[Means for Solving the Problems]
Accordingly, the present invention provides a substrate and a host layer on the substrate, the first ferromagnetic film, a non-ferromagnetic spacer film on and in contact with the first ferromagnetic film, and the spacer film A host layer comprising: a second ferromagnetic film in contact with the first ferromagnetic film, wherein the second ferromagnetic film is antiferromagnetically exchange-coupled to the first ferromagnetic film across the spacer film; A magnetic recording medium comprising a magnetic recording layer on the second ferromagnetic film, the magnetic recording layer having a composition different from the composition of the second ferromagnetic film.
[0008]
In a preferred embodiment, the thickness and material of the two ferromagnetic films are selected such that moments from the individual ferromagnetic films are essentially cancelled. Therefore, the host layer does not have a net magnetic moment or has a very small non-zero moment and therefore does not contribute to the Mrt of the magnetic recording layer.
[0009]
The magnetic recording layer has a composition different from that of the upper ferromagnetic film in the host layer, and is ferromagnetically coupled to the upper ferromagnetic film of the host layer. The magnetic volume V of the composite structure (magnetic recording layer and host layer) that determines thermal stability is approximately the sum of the volume of particles in the magnetic recording layer and the volume of the AF-coupled ferromagnetic film in the host layer. However, the magnetic moment of the composite structure is only the moment from the magnetic recording layer. This is because the host layer is designed to have essentially no net magnetic moment. Thus, antiferromagnetic coupling between the two ferromagnetic films of the host layer provides a mechanism to increase the effective thickness of the composite structure without increasing the net Mrt value of the composite structure.
[0010]
In an alternative embodiment, the two AF coupling films of the host layer are still antiparallel oriented, but have magnetic moments that are intentionally different in size so that the host layer has a net magnetic moment. This can be done to optimize the recording performance, reduce thermal decay, or design the media to a certain value of magnetic moment and coercivity without changing the manufacturing process.
[0011]
For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description of embodiments thereof taken together with the accompanying figures.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a cross-sectional layer structure of a prior art disk 10 having a single layer Co alloy magnetic recording layer 15. To form the data recording area of the disc, a thin film layer is sputter deposited on at least one of the substrates 11, preferably both flat surfaces. The disk substrate 11 can be made of any suitable material such as glass, SiC / Si, ceramic, quartz, or an AlMg alloy base with a NiP surface coating. The seed layer 12 is an optional layer that can be used to improve the growth of the lower layer 13 and thus magnetic properties such as the coercivity of the magnetic layer 14. The seed layer 12 is most commonly used when the substrate 11 is non-metallic such as glass. The seed layer 12 has a thickness in the range of about 5-50 nm and is known in the prior art to be effective as a seed material that promotes growth in several preferred crystal orientations of the later deposited layer, One of materials such as CrTi and NiAl. The lower layer 13 is a nonmagnetic material such as chromium or a chromium alloy such as CrV or CrTi, which is deposited directly on the seed layer if present, otherwise directly on the substrate 11. When the thickness of the lower layer 13 varies, the magnetic properties of the magnetic layer 15 such as the coercive force change. The lower layer 13 has a thickness in the range of 1 nm to 10 nm, and a typical value is about 20 nm.
[0013]
Prior to the deposition of the magnetic layer 15, a very thin (typically 0.5-5 nm) Co alloy onset or nucleation layer 14 is deposited on the lower layer 13. Nucleation layer 14 has a composition selected to enhance the growth of hexagonal close-packed (HCP) Co alloy magnetic layer 15 such that the C-axis is oriented in the plane of the layer. The nucleation layer 14 may be a CoCr alloy having a Cr composition layer selected to make the nucleation layer 14 non-ferromagnetic or very slightly ferromagnetic. Alternatively, the nucleation layer 14 may be a ferromagnetic Co alloy, in which case the nucleation layer 14 affects the magnetic properties of the magnetic layer 15. The Co alloy magnetic recording layer 15 may be a CoPtCrB alloy containing 4 to 25 atomic% of platinum, 10 to 23 atomic% of chromium, and 2 to 20 atomic% of bromine. When the magnetic layer 15 is CoPtCrB, the nucleation layer 14 may be CoPtCr or CoPtCrB having less than 6 atomic% B. The magnetic layer 15 generally has a thickness in the range of 5 to 20 nm.
[0014]
The protective overcoat 16 may be a conventional overcoat consisting of essentially amorphous carbon, optionally doped with hydrogen and / or nitrogen. The overcoat is generally less than 15 nm thick. All the layers from seed layer 12 to overcoat 16 described above are sputtered in an in-line sputtering system such as a commercially available single disk system having multiple sputtering target capacities or in a continuous process of a single disk system. can do. Sputter deposition of each layer can be performed with the above modifications using standard targets and techniques well known to those skilled in the art.
[0015]
[Preferred embodiment]
The magnetic recording medium of this embodiment has a magnetic recording layer formed on a special multilayer host layer. The host layer includes at least two ferromagnetic films that are antiferromagnetically (AF) exchange-coupled to each other with a non-ferromagnetic spacer film interposed therebetween.
[0016]
In a preferred embodiment, the two AF coupling films of the host layer have magnetic moments that are approximately equal in size but oriented antiparallel so that the host layer has substantially no net magnetic moment. However, the host layer may have some non-zero net magnetic moment because it is difficult to produce a film to the correct thickness.
[0017]
A magnetic recording disk 20 according to the present invention is shown schematically in cross section in FIG. In this figure, the host layer 30 replaces the onset or nucleation layer 14 of the prior art structure of FIG. As schematically shown in FIG. 2, the recording layer 25 is deposited on the host layer 30. The host layer 30 is made of two ferromagnetic films 32 and 34 separated by a non-ferromagnetic spacer film 36. The non-ferromagnetic spacer film 36 is selected such that the magnetic moments 42, 44 of adjacent films 32, 34 are AF coupled through the non-ferromagnetic spacer film 36, respectively, and are anti-parallel at zero applied magnetic field. Thickness and composition. Ferromagnetic films 32 and 34 has a magnetic moment value of Mr ₁ t ₁ and Mr ₂ t _2, respectively, Mr ₁ t ₁ and Mr ₂ t ₂ are approximately equal. (Since the residual magnetization Mr is expressed as a magnetic moment per unit volume of the ferromagnetic material, the product Mrt is a magnetic moment per unit area of the magnetic layer having a thickness t).
[0018]
In a preferred embodiment, each ferromagnetic film 32, 34 has substantially the same thickness t and is made of substantially the same ferromagnetic material so as to have the same Mr. Accordingly, the magnetic moments 42 and 44 substantially compensate or cancel each other, and the host layer 30 has substantially no net magnetic moment.
[0019]
AF coupling of ferromagnetic films by non-ferromagnetic transition metal spacer films has been studied in detail and described in the literature. In general, exchange coupling oscillates from ferromagnetic to antiferromagnetic as the thickness of the spacer film increases. This vibration coupling relationship in selected material combinations is described by Parkin et al., “Oscillations in Exchange Coupling and Magnetoresistance in Metallic Superlattice Structures: Co / Ru, Co / Cr and Fe / Cr” (Phys. Rev. Lett , Vol 64, p. 2034 (1990)). This material combination includes a ferromagnetic film made of Co, Fe, Ni and alloys thereof such as Ni—Fe, Ni—Co, Fe—Co and ruthenium (Ru), chromium (Cr), rhodium (Rh), iridium ( Ir), copper (Cu), and alloys thereof such as non-ferromagnetic spacer films. For each such combination of materials, the vibration exchange coupling relationship must be determined if it is not yet known, so the thickness of the non-ferromagnetic spacer film is the antiferromagnetic strength between the two ferromagnetic films. Selected to ensure magnetic coupling. The period of vibration depends on the non-ferromagnetic spacer material, but the strength and phase of vibration coupling also depend on the ferromagnetic material and interface quality. The vibrational antiferromagnetic coupling of ferromagnetic films is a spin valve type giant magnetoresistive (in order to design a continuous magnetization antiferromagnetic coupling film whose magnetic moment is strongly coupled antiparallel during head movement ( GMR) recording heads. These types of spin valve structures are described in US Pat. Nos. 5,408,377 and 5,465,185. Two ferromagnetic films used in a spin valve head and antiferromagnetically coupled across a very thin non-ferromagnetic spacer film as shown in the host layer 30 structure of FIG. This type of magnetic structure is also called "synthetic antiferromagnetic". If the structure does not have a net magnetic moment because the moments from the individual ferromagnetic films are canceled out, this structure can be referred to as a “compensated” synthetic antiferromagnetic magnet.
[0020]
In this AF coupling structure of the host layer 30, the orientations of the magnetic moments 42, 44 of the adjacent films 32, 34 are aligned antiparallel and are therefore added together to weaken each other. Arrows 42 and 44 indicate the directions of moments of individual magnetic domains directly above and below the AF coupling film 36. In the absence of an applied magnetic field, the lower ferromagnetic film 34 on the lower layer 23 has a grain structure in which the moments of the individual magnetic domains are essentially randomly oriented in the film plane. The particles of the ferromagnetic film 32 form a magnetic domain having a moment orientation that is antiparallel to the moment orientation of the ferromagnetic film 34 that directly faces the AF coupling film 36.
[0021]
The type of ferromagnetic material and the thickness values t ₁ , t ₂ of the ferromagnetic films 32, 34 are selected such that the net moment at zero applied magnetic field is essentially zero. The Mrt of the host layer 30 is given by Mr ₁ t ₁ -Mr ₂ t ₂ . In a preferred embodiment, Mr ₁ t ₁ should be equal to Mr ₂ t ₂ . This is accomplished by using the same ferromagnetic material for the two films 32, 34 and making t _{1 the} same as t ₂ . If different ferromagnetic material compositions are used for the two films 32, 34 such that the magnetization (magnetic moment per unit volume of the material) of the two ferromagnetic films is different, the thickness is accordingly Adjusted. Although FIG. 2 shows a host layer 30 having a two-film structure consisting of a single spacer film, the present invention can be extended to a host layer structure having a plurality of spacer films and a plurality of ferromagnetic films.
[0022]
The magnetic recording layer 25 is deposited directly on the upper ferromagnetic film 32 that also serves as an onset or nucleation layer for the magnetic layer 25. Since the host layer 30 is intended to function as a compensated synthetic antiferromagnetic magnet, the composition of the magnetic layer 25 on top of the film 32 must be different from the composition of the film 32. In a preferred embodiment, the magnetic layer 25 is a CoPtCrB alloy, and the ferromagnetic films 32 and 34 of the host layer 30 are also CoPtCrB alloys, but have different compositions, for example, B is present in an amount sufficiently smaller than the magnetic layer 25. . Alternatively, the films 32 and 34 may be a CoCr alloy or a CoPtCr alloy. The films 32 and 34 may have a thickness in the range of 0.5 to 5 nm. The non-ferromagnetic spacer film 36 in the host layer 30 is a 0.6 nm Ru film. This thickness of the Ru spacer film was selected to be the first antiferromagnetic peak in the vibration coupling relationship. It may also be desirable for each of the CoPtCrB ferromagnetic films 32, 34 to include an interface film consisting essentially of 0.5 nm Co at the interface with the Ru spacer film 36. These ultrathin Co films increase the interfacial moment between the ferromagnetic film and the spacer film, resulting in enhanced antiferromagnetic coupling. However, antiferromagnetic exchange coupling occurs without incorporating a Co interface film in the CoPtCrB ferromagnetic films 32 and 34.
[0023]
The upper ferromagnetic film 32 is ferromagnetically exchange coupled to the magnetic layer 25 and is weakly antiferromagnetically coupled to the lower ferromagnetic film 34 with the spacer film 36 interposed therebetween. When the magnetic field from the write head switches the magnetization direction of the particles in the magnetic layer 25, the magnetization direction of the upper film 32 also switches because these particles are exchange coupled. Since the magnetization direction of the lower film 34 is weakly antiferromagnetically coupled to the upper film 32, it is switched. Therefore, the moments of the films 32 and 34 remain antiparallel regardless of the magnetization direction of the particles in the magnetic layer 25 above the host layer 30.
[0024]
The thermal stability of the composite structure of the preferred embodiment (magnetic layer 25 and host layer 30) is increased compared to a single magnetic layer, even though the magnetic moments of films 42 and 44 are antiparallel. This is because the anisotropy of the particles in 32 and 34 is almost uniaxial and can therefore be added to strengthen. The magnetic volume V of the composite structure that determines thermal stability is approximately the sum of the volume of particles in the magnetic layer 25 and the AF coupling films 32 and 34. However, since the host layer 30 essentially has no net magnetic moment, the magnetic moment of the composite structure is only the magnetic moment from the magnetic layer 25. Antiferromagnetic coupling between the two ferromagnetic films 32, 34 provides a mechanism to increase the effective film thickness of the composite structure while reducing the net Mrt value of the composite structure. Therefore, the ferromagnetic film can contain particles having a very small diameter, and the thermal stability can be maintained.
[0025]
In an alternative embodiment, the two AF coupling films of the host layer are still antiparallel oriented but intentionally have different magnetic moments and the host layer has a net non-zero magnetic moment (Mr ₁ t ₁ is not equal to Mr ₂ t ₂ ). One reason for such an embodiment is that the optimum recording performance level of this structure can occur in that the thickness of the lower ferromagnetic film in the host layer is not equal to the thickness of the upper ferromagnetic film. is there. If the same material is used for both of these films (Mr ₁ = Mr ₂ ), Mr ₁ t ₁ will not be equal to Mr ₂ t ₂ . It is necessary to maintain the top film of the host layer at a thickness that does not cause recording noise to the magnetic recording layer, and it is also necessary to make the bottom film of the host layer sufficiently thick to develop the desired in-plane C-axis orientation more strongly There may be some cases. The second reason for this alternative embodiment relates to the thermal stability of the media. Recording magnetic transitions in the medium generate a magnetic field in a direction that tends to demagnetize adjacent transitions and thus facilitate thermal decay. In the host layer of the present invention, a transition occurs in the low ferromagnetic film that creates a magnetic field that is opposite to this demagnetizing field and thus reduces thermal decay. A third reason for this alternative embodiment is to adjust the signal level or Mrt of the media. It is common in the disk drive industry to design disks according to required values of Mrt or coercivity (H _c ). However, Mrt and H _c are correlated. Therefore, if the Mrt values are given, the conventional approach to reach the H _c design point of the medium, but by varying the processing conditions such as the lower layer thickness and deposition temperature, it is possible deleterious to the recording properties possible. According to the present invention, Mrt can be adjusted by changing in different ways, i.e. the relative values of Mr ₁ t ₁ and Mr ₂ t _2.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view of a prior art magnetic recording disk having a single magnetic recording layer.
FIG. 2 is a schematic cross-sectional view of a magnetic recording disk according to an embodiment of the present invention.

Claims (8)

A substrate, a host layer on the substrate, a first ferromagnetic film, a non-ferromagnetic spacer film on and in contact with the first ferromagnetic film;
A second ferromagnetic film on and in contact with the spacer film, wherein the second ferromagnetic film is antiferromagnetically exchanged with the first ferromagnetic film across the spacer film A combined host layer,
A magnetic recording layer on the second ferromagnetic film of the host layer, the magnetic recording layer having a composition different from the composition of the second ferromagnetic film ,
The first ferromagnetic film has a thickness T1 and a magnetization M1, the second ferromagnetic film has a thickness T2 and a magnetization M2, and the first ferromagnetic film and the second ferromagnetic body Magnetic recording media having different magnetic moments (M1 × t1) and (M2 × t2) per unit area of the film . The spacer film of the host layer is formed of a material selected from the group consisting of ruthenium (Ru), chromium (Cr), rhodium (Rh), iridium (Ir), copper (Cu), and alloys thereof. The magnetic recording medium according to claim 1 . Wherein a first ferromagnetic film and the second ferromagnetic film is Co, Fe, Ni, and manufactured by material selected from the group consisting of an alloy of the host layer, any one of claims 1 to 2 The magnetic recording medium according to one item. The first ferromagnetic film of the host layer includes an interface film consisting essentially of cobalt placed on the interface of the first ferromagnetic film and the spacer film, any one of claims 1 to 3 The magnetic recording medium according to one item. The second ferromagnetic film of the host layer includes an interface film consisting essentially of cobalt placed on the interface of the second ferromagnetic film and the spacer film, any one of claims 1 to 4 The magnetic recording medium according to one item. Said magnetic formed on the recording layer further comprises a protective overcoat was, magnetic recording medium according to any one of claims 1 to 5. The magnetic recording medium according to claim 1 , wherein the first ferromagnetic film and the second ferromagnetic film are formed of the same material, and T1 is different from T2. 2. The magnetic recording medium according to claim 1 , wherein the first ferromagnetic film and the second ferromagnetic film are formed of different materials, and T <b> 1 and T <b> 2 have substantially the same thickness.

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